This invention relates generally to ferrite circulators used in microwave circuitry, and more particularly, to ferrite circulators for coupling microwave electromagnetic energy between microstrip lines. The principles of ferrite circulators are well known, and documented in a number of texts, for example, "Microwave Ferrites and Ferrimagnetics," by Benjamin Lax and Kenneth J. Button, published by McGraw-Hill (1962).
In a three-port ferrite circulator, there are three transmission paths spaced radially about a generally cylindrical ferrite element subject to an appropriate magnetic field. Electromagnetic energy transmitted toward the ferrite element along a first path is transmitted out along the next adjacent path, spaced 120.degree. from the first. The transmission paths may be electromagnetic waveguides, or may be microstrip lines consisting of a metallic strip spaced from a ground plane by a dielectric layer.
Circulators have wide application in very-high-frequency communications systems. In particular, there are increasing numbers of communications systems and subsystems being produced using integrated circuits for operation in the millimeter-wave communications frequency band. At frequencies above 20 gigahertz (GHz), there has been a critical problem in developing such circuits because of the lack of an acceptable circulator.
The principal difficulty encountered in producing an acceptable circulator for use with microstrip lines at these high frequencies is the difficulty of matching the microstrip lines to the ferrite. Electrical impedance matching dictates that a relatively wide microstrip line be used, to match the relatively low impedance of the ferrite. However, use of a wide microstrip line results in a substantial mismatch in magnetic coupling properties of the microstrip lines and the ferrite. If the microstrip lines are appropriately matched with the ferrite to provide the proper coupling angle, by using relatively narrow microstrip lines, there is a serious mismatch in electrical impedances.
Prior to this invention, no circulators of the prior art had successfully addressed this problem. Other circulators have provided proper electrical impedance matching over a narrow range of frequencies, and a proper coupling angle with the ferrite such that desired performance is obtained over a different narrow range of frequencies. However, prior to this invention it has not been possible to provide proper coupling with the ferrite in a structure that also provides electrical impedance matching over a common wide range of frequencies. Some attempts to solve this problem have focused on making changes to the ferrite properties, or to its size, to minimize the degree of mismatch, but none has been successful.
It will be appreciated from the foregoing that there has been a need for a microstrip circulator structure that provides for both magnetic and electrical matching of the microstrip lines to the ferrite element. The present invention achieves this end and is well suited for fabrication using integrated circuit techniques.